Bright gold plating process



United States Patent 3,14%,053 BRIGHT GGLD PLATHJG PROCESS Edward A. Parker, Qranston, and James A. Powers, East Providence, 5.2.1., assignors to Technic, ind, Cranston, R1,, a corporation of Rhode Island No Drawing. Filed Dec. 31, 1959, Ser. No. 863,111 8 Claims. (Cl. 204-46) The present invention relates to gold plated products and to a process for electrodepositing gold and gold alloys and more particularly to the electrolytic preparation of gold and gold alloy plates presenting hard, heat-resistant, smooth, mirror-bright surfaces.

For many years, it has been common practice in the electrodeposition of gold to use a bath consisting of potassium gold cyanide, along with other alkali or alkali producing materials. These ancillary products were included in the bath for various reasons, such as to increase conductivity; to obtain good throwing power; to maintain color; or to buffer the bath on the alkaline side.

Among these products may be mentioned free alkali cyanide, alkali metal hydroxides and carbonates, and, almost always, a di-basic or tri-basic alkali metal phosphate.

These gold baths must be operated at elevated temperatures, and are usually limited to rather low current densities. It is also difficult to obtain bright deposits in heavy thickness, i.e. greater than 0.000050 inch, which will be hard, heat-resistant, bright, etc. 7

Gold and gold alloy baths have always been operated on the alkaline side, however, for reasons which are well founded. For instance, most gold baths contain free cyanide as an aid to throwing power. Consequently, these baths must remain alkaline or the extremely toxic hydrogen cyanide gas would be generated in situ and released from the bath. Still another reason is that soluble metal aurocyanide solutions are unstable with respect to strong mineral acids, as one cyanide radical is displaced from the complex and there separates from solution insoluble gold cyanide.

In recent years, the manufacture of printed circuits has created an expanding demand for a gold plating process which would utilize solutions not detrimental to the plastic laminate used as the base for the printed circuit. Heat and alkali are two of the most severe agents in the deterioration of this material.

We have found that the addition of a nickel (II) (nickel of valence l1), chelate of the aminopolycarboxylic acid type having a stability constant of the order of magnitude of about 10, i.e., stability of the order of that of nickel ll nitrilo triacetic acid, to an alkali metal aurocyanide solution at a relatively low pH will produce an electrodeposit of gold, which is more than 99.8% pure gold, i.e. 24K gold. The amount of nickel chelate employed in the solution may range up to grams of nickel per liter. Gold content of the bath is preferably in the range from about 2 grams per liter to about saturation, with about 8 grams per liter preferred.

However, we prefer to operate in the region of 1-5 grams of nickel per liter. Heavy deposits of gold may also be obtained which are brilliant in character. The speed of plating is also increased so that current densities of 60 amperes per square foot are possible. Furthermore, the plates so obtained are of high reflectivity, increased hardness generally at least 120 Knoop, and exhibit better wear resistance, and tarnish resistance than gold deposited from conventional aurocyanide baths. That is, with respect to tarnish, the gold plate we obtain is so hard and heat resistant that it can withstand temperatures of 300 C. for periods of two hours and more without discoloration.

Accordingly, it is a primary object of the present invention to provide a bath from which gold may be electrodeposited without the harmful effects of alkali and alkali producing materials.

It is a further object of the present invention to provide a solution from which gold may electrodeposit to produce an electroplate of high reflectivity and which will provide the mirror image of an object placed before it.

It is a further object of the present invention to provide a solution from which gold may be electrodeposited in a smooth fashion at room temperature.

It is a further object of the present invention to provide a solution from which gold may be electrodeposited to give bright, lustrous, thick plates at fast plating speeds, which plates will show a high Knoop hardness, at least about 120, and temperature resistance of about 300 C. for two hours.

Other objects and advantages of the invention will in part be obvious and in part appear hereinafter.

The present invention, accordingly, essentially consists of an improved plate or article, characterized by its I bright, smooth, hard, pure gold plate and a bath from which such gold may be electrolytically deposited to form the article, the bath comprising an aqueous solution of an alkali metal aurocyanide with or without additional alloying metal compounds, the nickel (Il) (nickel, valence II), chelate of an appropriate amino polycarboxylic acid, preferably one selected from the group consisting of nitrilo triacetic acid, ethylene diarnine tetraacetic acid, fl-hy droxyethylethylene diamine triacetic acid, and ethylene diamine diacetic acid, and at least one buffer material to maintain the pH between 3.5 and 5.5 during the deposition of the gold from the solution. While generally we use a single chelate of this group, it is perfectly appropriate to use mixtures of the nickel chelates.

Since the brightening action of the nickel (II) chelate has been found to be a function of pI-Lit is essential that this parameter be controlled. The maintennace of the pH value of the plating bath between 3.5 and 5.5 is most easily accomplished by incorporation into the bath of salts of moderately strong organic acids whose acid dissociation constants are of such a nature that they serve as buffers in restricting the pH of the solution to limits as defined during the electrodeposition of the gold or its alloy. Among these substances may be listed, as representative, certain organic acids which have at least one ionization constant of such a value as to create a buifer range in the specified pH range, when properly neutralized with the appropirate amount of base.

In general, the acids most useful are the aliphatic acids containing 2 to 8 carbon atoms, and also may carry hy droXyl groups in functional relationship to the carboXylic groups. Typical useful acids are acetic, glycollic, lactic, citric, gluconic, tartaric, kojic.

This list, while not all inclusive of the substances which may be employed, is indicative of the general kinds of acid compounds which have been found to be useful. It is also possible to use mixtures of the acids, provided the pH of the final gold bath lies in the range of 3.5 to 5.5.

The electrolytic deposition can be carried out using insoluble anodes such as platinum, gold, stainless steel, or carbon. Replenishment of the gold and its alloying components can be effected by any convenient means, such as ampere-minutes, as is well known in the art. The ratio of anode to cathode surface area, while not critical, should be most preferably at least 1:1. The temperature of the bath should be held within the limits of 60 to F. during the electrodeposition as a preferred operating range. It has been found that mechanical agitation of the bath, such as with a stirrenserves to facilitate clean, smooth deposition, and at the same time permits the use of higher current densities without smutting the pleated surface. The

. Q9 electrical tension between the anode and the wares will usually lie between 2 and 6 volts.

The following examples are typical of the types of solutions from which hardplates having the properties indicated may be detained;

Example 1 Into an amount of water sufiicient to form one liter of solution is dissolved:

8 grams potassium gold cyanide 30 grams monopotassium phosphate 35 grams tri-potassium citrate 25 grams citric acid I 3.0 grams nickel as nickel (II), N,N'-ethylenediaminediacetate The pH of this solution may be adjusted to a value between 3.2 and 4.4 with'either phosphoric acid or potassium hydroxide. The electrodeposition of gold is then accomplished in the conventional manner. The bath temperature may lie from 70 F. to 120 F. The plate ob tained on brass, or other basis metal, with or without barrier layers of other plated metals, is hard, mirror bright, and tarnish resistant at temperatures of the order of 300 C. for two to three hours.

Example ll Into an amount of water suflicient to form one liter of solution is added:

12 grams of potassium gold (I) cyanide 60 grams acetic acid and potassium acetate 30 grams monopotassium phosphate 3 gramsnickel as potassium nickel (ll) nitrilotriacetate The pH should be adjusted to a value between 3.0 and 5.0 with phosphoric acid or potassium hydroxide. Thick, bright, lustrous electrodeposits of substantially pure 24K gold are obtained from this bath at current densities up to 60 amperes per square foot. Current efficiencies of 60% are also realized.

The plate obtained on any basis metal, whether over barrier layers or not, is mirror bright, hard, tarnish resistant at temperatures of the order of 300 C. for 2-3 hours. That is, in terms of this process, the basis metal is that which is in direct contact with the gold layer, whether the basis thus in contact be massive piece or mere layer.

Samples of gold plate, plated under conditions of EX- arnples I and II, were analyzed spectrographically by an independent laboratory and the following results reported:

Percent Gold 99.78 Nickel 0.18

By spectrographic analysis:

Silver 0.007 Cobalt 0.005 Copper 0.01 Iron 0.02 Manganese 0.001 Silicon 0.001

Knoop hardness tests of samples of the plate made according to Examples 1 and II consistently showed values of 120-160, for plates as thin as could be measured up to 2-3 mils (i.e. 0002-0003 inch).

Heating tests of the samples showed they remained mirror bright, in thickness approaching a flash (0.000003 inch) up to 1 and 2 thousandths inch after 2 and 3 hours at 300 C. Specimens of heavy plates, i.e., those up to 2-3 thousandths remained mirror bright at temperatures of 400-500" C. even after 4 hours. This, of course, emphasizes the unusual property of these gold plates when compared with other so called bright gold plates which will tarnish at 200 C. in 10 minutes.

For formulating other solutions in accordance with these examples, any of the nickel chelates mentioned or any combination of the chelates as a mixture may be used.

Though the invention has been described with reference to only a few examples, it is to be understood that variants thereof may be developed without departing from its spirit or scope.

We claim:

1. A method of electrodepositing bright, hard gold in substantially pure form which comprises electrolyzing a solution comprising a weak, stable, organic acid in amount sufficient to provide a pH of 3-5, goldas an alkali metal gold cyanide, and a nickel (H) chelate of an aminopolycarboxylic acid chelating agent, said nickel being in a substantially non-ionized form, and being retained in solution without significant deposition under plating conditions.

2. A method as claimed in claim 1, in which the nickel chelate is the 1:1 chelate of ethylenediarninetetraacetic acid.

3. A method as claimed in claim 1, in which the nickel chelate is the 1:1 chelate of nitrilotriacetic acid.

4. A method as claimed in claim 1, in which the nickel chelate is the 1:1 chelate of hydroxyethylethylenediarninetriacetic acid.

5. A method as claimed in claim 1, in which the nickel chelate is a mixture of the 1:1 chelate of nitrilotriacetic acid and ethylenediaminetetraacetic acid.

6. A method as claimed in claim 1, in which the nickel chelate is a mixture of the 1:1 chelate of ethylenediaminetetraacetic acid and nitrilotriacetic acid.

7. An electrolyte for depositing bright,- heat resistant substantially pure gold coatings on surfaces comprising a weak, stable, organic acid in amount sufiicient to provide a pH of 3-5, gold as a gold cyanide, a nickel (II) chelate of a compound selected from the group consisting of: nitrilotriacetic acid, ethylenediaminetetraacetic acid, fi-hydroxyethylethylenediaminetriacetic acid, ethylenediaminediacetic acid and mixtures thereof, said nickel being in a substantially non-ionized form, retained in solution Without significant deposition under plating conditions.

8. An electrolyte for depositing bright, heat resistant, gold coatings of substantially 24K fineness on surfaces comprising an organic acid in amount suflicient to provide a pH of 3-5, about 2 grams per liter to saturation of gold as potassium gold cyanide, about 1-10 grams per liter of nickel (II) as nickel chelate of an aminopolycarboxylic acid selected from the group consisting of: nitrilotriacetic acid, ethylenediaminetetraacetic acid, ,B-hydroxyethylethylenediaminetriacetic acid, ethylenediarninediacetic acid and mixtures thereof, said nickel being in a substantially non-ionized form and retained in solution without significant deposition under plating conditions.

References Cited in the file of this patent UNITED STATES PATENTS 2,724,687 Spreter et al. Nov. 22, 1955 2,905,601 Rinker et al. Sept. 22, 1959 2,967,135 ()strow et al. Jan. 3, 1961 

1. A METHOD OF ELECTRODEPOSITING BRIGNT, HARD GOLD IN SUBSTANTIALLY PURE FORM WHICH COMPRISES ELECTROLYZING A SOLUTION COMPRISING A WEAK, STABLE, ORGANIC ACID IN AMOUNT SUFFICIENT TO PROVIDE A PH OF 3-5, GOLD AS AN ALKALI METAL GOLD CYANIDE, AND A NICKEL (II) CHELATE OF AN AMINOPOLYCARBOXYLIC ACID CHELATING AGENT, SAID NICKEL BEING IN A SUBSTANTIALLY NON-IONIZED FORM, AND BEING RETAINED IN SOLUTION WITHOUT SIGNIFICANT DEPOSITION UNDER PLATING CONDITIONS. 